What Is Spaced Repetition? The SM-2 Algorithm Explained Simply

You've probably had the experience of studying for a test, acing it, and then forgetting everything two weeks later. This isn't your fault — it's how human memory works. But there's a technique that directly fights this: spaced repetition.

Spaced repetition is used by millions of language learners worldwide, and it's the engine behind apps like Anki, TheLernen, and parts of Duolingo. Once you understand how it works, you'll never study vocabulary the same way again.

The Forgetting Curve: Why You Forget

In 1885, German psychologist Hermann Ebbinghaus ran experiments on himself to measure memory. He discovered that forgetting follows a predictable curve: within 20 minutes of learning something, you've already forgotten about 40% of it. After a day, you've forgotten 67%. After a week, about 75%.

This is the "Ebbinghaus Forgetting Curve." The steep initial drop explains why cramming the night before an exam feels effective but leads to rapid forgetting. Your brain simply hasn't had time to consolidate the memory into long-term storage.

How Spaced Repetition Fights the Forgetting Curve

The key insight is that every time you successfully recall a memory just before forgetting it, two things happen: the memory gets stronger, and the time before the next "forgetting point" gets longer.

If you learn a word today and review it tomorrow, next week, in three weeks, and in two months — each successful review pushes the next review interval further into the future. Eventually, you've seen the word enough times that it moves into long-term memory and needs only very occasional review to stay fresh.

The SM-2 Algorithm: How It Actually Works

The SM-2 (SuperMemo 2) algorithm, developed by Polish researcher Piotr Woźniak in 1987, is the mathematical formula most spaced repetition apps use. Here's the simplified version of what happens:

• 1. You see a flashcard and rate how well you remembered it (e.g. "Again", "Hard", "Good", "Easy")
• 2. The algorithm assigns a score from 0–5 based on your rating
• 3. It calculates your "easiness factor" — how naturally this word sticks in your memory
• 4. It sets the next review interval: e.g. 1 day → 6 days → 15 days → 35 days...
• 5. If you rate "Again" (forgot it), the interval resets and starts over
• 6. The more consistently you recall a word, the longer the intervals grow

Spaced Repetition vs. Traditional Flashcards

Traditional flashcards work on a simple "shuffle and repeat" basis — every card gets shown in random order, regardless of whether you know it well or poorly. This wastes time on words you already know and doesn't review difficult words often enough.

How TheLernen Uses SM-2

TheLernen implements the SM-2 algorithm for all 4,000 vocabulary words across German, English, Japanese and Polish. After each flashcard session, you rate each word on a 4-point scale: Again / Hard / Know / Easy.

• 🔴 Again — you didn't remember it. The interval resets to 1 day.
• 🟠 Hard — you remembered, but it was difficult. Interval grows slowly.
• 🟢 Know — you remembered it well. Interval grows normally.
• 🔵 Easy — it was immediate. Interval grows faster.

The app tracks each word's interval independently, so your daily review queue contains exactly the words that are due — no more, no less. A 20-minute daily session covers everything you need to review to maintain 85%+ retention.

Practical Tips for Using Spaced Repetition

• ⏰ Review daily, not in big weekly sessions — consistency beats intensity
• 🎯 Be honest with your ratings — marking "Easy" when it was actually hard defeats the algorithm
• 📦 Keep sessions small: 15–30 new words per day maximum when starting
• 🌅 Morning sessions work best — memory consolidation peaks after sleep
• 🔁 Don't skip days — a missed day means words pile up and the next session feels overwhelming